European patent publication EP 1 867 610 in the name of Draka Comteq B.V. discloses such an apparatus for manufacturing an optical fiber. In the plasma activated chemical vapour deposition (PCVD) process, deposition is performed on the inside of a substrate tube. In this process, the resonator is fed by a microwave source (typically a magnetron). Typically, the resonator moves back and forth in the axial direction of the substrate tube. Inside the substrate tube, the microwave power creates a plasma which activates a reaction resulting in deposition of thin quartz layers within the substrate tube. The substrate tube and resonator are placed inside a furnace.
When inspecting the deposition inside the substrate tube, a non-uniform thickness and/or refractive index is observed along the axial direction of the substrate tube. This phenomenon has strong negative impact on some resulting fiber quality parameters such as the attenuation (OTDR traces) and/or the uniformity of mode-field diameter for single mode fiber and/or the uniformity of the alpha-value for multi-mode fiber. The non-uniformity of the deposition thickness includes near periodic variations.
The cause of these variations is the microwave interaction of the resonator with the surrounding, mainly dominated by the metals shell around the furnace, which is needed for construction purposes and for prevention of high microwave leakage for health safety reasons. The effect of the microwave interaction becomes even worse when the amount of deposited glass increases due to a cumulative effect and due to limited adaptive capabilities of the microwave configuration, like the efficiency of the chokes. There can also be lengthwise deposition non-uniformity near the ends of the substrate tube due to reflections of microwave power, guided by the plasma at the tube, at the ends of the furnace. A non-uniform microwave power along the length of the substrate tube causes a length/front position, relative to the resonator position, to change along the length of the substrate tube, thus introducing deviation to deposition targets based on the position of the resonator.
In order to reduce the variations in the deposition thickness, patent publication US 2009/0022906 discloses moving the furnace along the axial direction of the tube. Results show that this approach is effective to a certain extend in smoothing the variations. When the variations are too high, for instance for very thick amount of glass depositions (e.g. CSA larger than circa 350 mm2) and/or very small inside diameters (e.g. smaller than circa 20 mm) and/or high deposition rate (e.g. larger than circa 2.5 g/min) or microwave power (e.g. more than circa 5 kW) and/or better quality requirement, a higher reduction factor may be desirable.
Furthermore, a moving furnace has an additional negative effect, because it decreases the effective core rod length by lowering the total resonator sledge pass length by a distance, which influences the fiber cost in a negative way.